I've now corresponded with several clients or potential clients who at least appear to be using signatures in their Message communications.
I can't find the settings for this anywhere, and I'm not interested in copy-and-pasting each time.
Am I missing something?
Didn't someone once say that messaging system signatures are the last refuge of a scoundrel?
Why would anybody WANT to use a signature here?
I think the proper form your question should be:
"How can I avoid the natural and understandable irritation that I'm inclined to feel when I see Upwork users using signatures?"